The present invention relates to a method for producing and checking a thread arranged in a cylinder head to receive a spark plug. However, the invention also relates to a device for checking the thread introduced into the cylinder head.
In an ideally operating internal combustion engine, combustion gas, i.e. the fuel-air mixture, is ignited by the spark plug. Conventional spark plugs have a central electrode and a ground electrode; the ground electrode can also be designated as a roof electrode. The latter has a vertical web extending substantially parallel to the central electrode, from which an ignition surface is bent over in the direction of the central electrode as a transverse web. The transverse web has the free front edge. The ignition surface is spaced apart axially with respect to an ignition end of the central electrode, so that a spark gap is formed, in which an ignition spark is generated by means of known means and coordinated with the injection time. It is thus possible for the combustion gas to be ignited, this being generally known.
The combustion gas, i.e. the fuel-air mixture, must have in this case a suitable mixture ratio in order to be able to ensure proper ignition and thus a desired combustion. As a result of tolerances and cycle (work cycle) to cycle (work cycle) deviations, the combustion gas can deviate from this optimal mixture ratio, which results in more or less stable ignition and/or combustion conditions. Here, in particular the volume located between the roof electrode and the central electrode at the ignition time is critical. If, during the period of ignition, there is no optimally ignitable mixture in the area of the spark gap, this can lead to a slow or incomplete combustion process, in the worst case even to a complete misfire.
Usually, the injector is set with its injection angle relative to the spark gap between the roof electrode and the central electrode such that the desired optimal, substantially lean fuel gas mixture can be ignited. For example, the fuel jet injected could deviate from its set injection angle, so that there is a too rich or too lean, non-optimally ignitable fuel gas mixture in the area of the spark gap between the roof electrode and the central electrode.
If a conventional spark plug is screwed into the cylinder head, the rotational orientation, in particular of the roof electrode and therefore the position of the spark gap, is absolutely random, which is of no further importance for internal combustion engines with homogeneous combustion. However, in internal combustion engines with direct injection, this means that the combustion process can vary, since the roof electrode almost conceals the spark gap in relation to the injector, so to speak, so that the actually achievable combustion quality is not present. This is also because the fuel does not evaporate during the direct injection but, following a rectilinear flight path, reaches the spark plug in droplet form, the spark gap thereof being concealed by the electrode. To this extent, as already mentioned above, misfiring, increased fuel consumption and reduced effectiveness with associated increased environmental pollution are to be expected if the fuel jet injected does not reach the area of the spark gap between the roof electrode and the central electorate as envisaged, so that even measures for adjusting the injection angle of the fuel jet are ineffective.
From this starting point, attempts are known to align the spark plug with its roof electrode oriented toward the fuel injector, so that the fuel jet injected can reach the spark gap unimpeded (oriented spark plug).
DE 10 2007 024 878 A1 proposes a spark plug, the spark plug body of which has a rotary code, the external thread being formed on a nut that can be rotated around the spark plug body. The mount for the spark plug has a rotary code that is complementary thereto. Thus, the spark plug might be able to be inserted only in a single orientation predefined by the complementary rotary code. Fixing is carried out by means of the nut without rotating the spark plug body.
EP 1 508 947 B1 likewise deals with measures for aligning the roof electrode, i.e. the spark gap, toward the injector. The spark plug has a deformable sealing and compensating element, which is arranged in such a way that the sealing and compensating element is loaded with a tightening torque as the spark plug body is screwed in, the sealing and compensating element being formed with such a geometry and from such a material that the sealing and compensating element is loaded up to its elastic limit when a predetermined tightening torque is applied and, as the spark plug is rotated further, the material thereof begins to flow above the yield point. A hexagon is formed on the spark plug body in such a way that the ground electrode has an accurate, determined and detectable position in relation to the mounting tool when the mounting tool for the spark plug is placed on the hexagon. The intention is to ensure that rotation beyond the prescribed tightening torque is possible, it being possible for the central electrode to be brought into the desired position via this additional rotation. In order to be able to determine the position of the ground electrode reliably, EP 1 508 947 B1 proposes the arrangement of a bi-hex structure, which can be assigned to the position of the ground electrode, on an outer surface of the hexagon, the central axis of the bi-hex structure being congruent with the central axis of the roof electrode, and the tip of the bi-hex structure equally being oriented with the extent of the roof electrode toward the central electrode.